qcast-front/.agents/skills/vercel-react-best-practices/rules/rerender-transitions.md

---
title: Use Transitions for Non-Urgent Updates
impact: MEDIUM
impactDescription: maintains UI responsiveness
tags: rerender, transitions, startTransition, performance
---

## Use Transitions for Non-Urgent Updates

Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.

**Incorrect (blocks UI on every scroll):**

```tsx
function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => setScrollY(window.scrollY)
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}
```

**Correct (non-blocking updates):**

```tsx
import { startTransition } from 'react'

function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0)
  useEffect(() => {
    const handler = () => {
      startTransition(() => setScrollY(window.scrollY))
    }
    window.addEventListener('scroll', handler, { passive: true })
    return () => window.removeEventListener('scroll', handler)
  }, [])
}
```